Direct-current dynamoelectric machine and ventilating system therefor



. LYNN 2 381,296 Aug. DIRECT-CURRENT DYNAMIC-ELECTRIC MACHINE AND VENTILATING SYSTEM THEREFOR Filed NOV. 7, 1942 2 Sheets-Sheet l INVENTOR v Clarence Lynn.

ATTORNEY WITNESSES:

' 1% i Clare/205 Lynn. g f w z E c. LYNN 2,381,296 DIRECT-CURRENT DYNAMO-ELECTRIC MACHINE AND VENTILATING SYSTEM THEREFOR 7 Filed Nov. 7, 1942 2 Sheets-Sheet 2 Aug. 7, 1945.

INVENTOIR ATTORNEY Patented Aug. 7, 1945 UNI TED STATES iA'IENT OFFICE v I DIRECT CURRENT IZZ MOELE CTR-1C MA- A CHINE THEREFOR AND VENTILATING SYSTEM Application November 7, 1942, Serial No. 464,874

9 Claims.

My invention broadly relates to dynamo-electric machines, but more particularly relates to reversible high-power low-speed direct-current mill-motors; mill-motors being the common designation applied to electric motors used in metal fabricating mills, especially steel mills, for driving roller-means by which metal is formatively treated, such as, for example, in steel-rolling.

Mill-motors of the type to which my invention more particularly pertains comprise salient-pole direct-current machines capable of operating at various speeds in either direction, but generally comprising rotors which operate at exceptionally low peripheral speeds of below about 5500 feet per minute, and considerably less for mill-motors having exceptionally low rotational speedsineluding such mill-motors having high power,

running up to from 7000 to 10,000 E. P., as present-day limits, which may even be exceeded. Such exceptionally high-power mill-motors customa'rily have base rotational-speeds of as little as 35 R. P. M. or less, the base rotational-speeds of mill-motors being generally, but not always, less than about 100 R. P. M. By base speed, I mean the lower R. P. M. value at which the motor is rated. For example, a 7000 H. P., 700 volts, 35 to '70 R. P. M., reversible direct-current millmotor is one having a rated base speed of 35 R. 1?. M. in either direction, with a recommended maximum rated speed of '70 R. P. M. At this point. it may be well to state that this motor, having 24 main poles, is further repeatedly referred to hereinafter as illustrative of an actual mill-motor embodying the teachings of my invention.

In order to deliver the required power at such slow speeds, which may be from several to several hundred horsepower per revolution per minute, mill-motors must be designed with high torques. In addition, a mill-motor of the type described usually must be capable of handling for short time overloads of from 2 to 2 times normal rated load, and of reversing from base speed in one direction to base speed in the reverse direction in approximately two seconds. Because of these and other factors, it has been found to be necessary to utilize large currents and large fluxes in such mill-motors. Consequently, the heat-losses developed in such a motor are largeelectrical efficiencies of high-power mill-motors of this kind being about 92%, more or less-so that the mill-motor must be provided with adequate ventilation.

However, mill-motors have certain definite physical limitations which have heretofore been considered as requiring a design with a relatively large diameter and a relatively short core length.

Among these physical limitations is the natural tendency in mill-motors, common to all direct-current dynamo-electric machines, for ventilating air to flow from the rear end to the front or commutator end of the machine because of the asymmetry introduced by the commutatorassembly. However, in a mill-motor the path for incoming air at the rear of the machine is restricted around the large rotor-shaft, such shafts having diameters running from a fraction of a foot for relatively lower power (600 H. P.) somewhat higher speed. machines R. PL'M. base speed) to a few feet for machines in the higher power, lower speed range. Such a shaft and the associated armature spider limit the entrance area or ventilating inlet for incoming air at the rear end of the motor, so that a larger motor diameter was thought to be the solution. Moreover, the output of a machine of the type described can be expressed as proportional to D L, where D is the outside diameter of the rotorcore punchings and L is the length of the stacks 'of core punchings, parallel to the shaft. Assuming that the most favorable permissible dimensions were arrived at for a given power, a power increase was not considered possible for a millmotor through increasing its core length alone.

The amount of ventilating air that can be passed through the restricted ventilating inlet or entrance area around the motor shaft has a'maximum for a given apparatus so that the increment of heat losses, which would be introduced by lengthening the core, could not be adequately removed without increasing the diameter so as to enlarge the opening or entrance area for the incoming ventilating air and the area for the stator air-passages.

Another physical limitation relating to the length of the core of a mill-motor arises out of the high magnetic flux used in such machines. The high average flux density demands a heavy solid or continuous stator-frame, a large number of highly excited main poles and a narrow airgap. The large number of poles are compactly arranged around the stator-frame for the purpose of reducing the proportion of low flux in the circumferential flux distribution curve for the air-gap, such low flux being found at the edges of the poles. These conditions result in concentrated stator heat-losses which can be removed only by the Ventilating air flowing [through the narrow air-gap and across the airgap into and through restricted stator ventilatsize of its diameter.

ing air-passages between adjacent closely spaced poles. Since only a limited amount of air can be forced through a long and narrow path, the core-length of the mill-motor is kept down in order to limit the length of the ventilating air paths for the ventilating air which coo-ls the stator parts.

By introducing. my invention, a mill-motor is provided having higher powers and lesser inertia than prior machines for a fixed core length. In accordance with my invention, air is brought into the mill-motors from both ends of the machine, around the rotor, and discharged from both ends of the stator. The ventilating air itself is taken from the general permeating atmosphere in the preferred construction. Although the pole and core axial lengths are increased, the distance over which air is compelled to flow axially in any one direction for cooling the stator is less because the air flows in opposite directions in the dlfierent end-halves of the air-gap and the stator air-passages. Generally, the air which flows into the mill-motor at either end comes out at the same end. The core of the machine can be lengthened for obtaining additional horsepower output at relatively lower rotor inertia for the output, other things being the same; or a machine of lower inertia for the same output can be obtained by maintaining the D L product, but increasing the core length at the expense of the My invention makes this possible because adequate ventilation is provided in spite of the natural obstructions which a machine such as a mill-motor introduces against such ventilation.

The diameter D and length L of the rotor or armature core determine its outer peripheral area which is traversed by all of the flux, carries the armature coils, and is subjected to ventilation. These dimensions also influence the rotational inertia of the rotor about its axis. When a motor of a given rotational speed and rating is redesigned with a smaller diameter and greater length, the

core diameter decreases in less proportion than the increase in core length, but the peripheral area is relatively increased and peripheral linear speed decreased. Rotational inertia may be said, for illustrative purposes, to be proportional approximately to the fourth power of the core diameter and the first power of the core length, so that, although the core diameter may be decreased in lesser proportion than the core length is increased, the rotational inertia can be materially decreased when the fractional decrease in the fourth power of the core diameter is greater than the fractional increase in the core length. The simple proportions and assumptions just made are for illustrating a general underlying principle, and no claim is made that they represent an accurate mathematical analysi or include all the factors involved in a strict treatment.

An object of my invention is to provide a millmotor with a, special ventilating arrangement which permits the mill-motor to be provided with a comparatively long core-length and a correspondingly relatively short core-diameter.

It is an object of my invention to provide a novel mill-motor having a ventilating system which permits the diameter of the armature or rotor-core to be decreased to the advantage of the operating characteristics of the motor, although the actual core-length is increased.

Another object of my invention lies in the provision of a high-power, low-speed, reversible, direst-current mill-motor having a rotor with a relatively low rotational inertia, so that the braking, acceleration and deceleration forces are minimized.

It is a further object of my invention to lower the cost of a mill-motor of the type described.

Other features, objects, innovations, and combinations of my invention will be discernible from the following description and from the attached drawings. In this description and in the drawings common motor-details are omitted or briefly described and shown in the interest of clarity and brevity.

In the drawings, which are not to scale:

Figure 1 is a longitudinal, part sectional, part elevational view of a mill-motor installation embodying my invention;

Fig. 2 is a transverse elevational view of a front end-housing of the mill motor;

Fig. 3 is a transverse partial vertical view of the mill-motor, with parts broken away to show the construction; and

Fig. 4 is an inside perspective view of part of the stator, illustrating the poles supported therey.

Referring to the drawings, a variable-speed direct-current mill-motor, indicated in entirety by the reference numeral 2, comprises an outer stator 4 and an inner rotor 6 which is supported by spaced bearing means 8.

The stator 4 comprises a single cylindrical motoror stator-frame II] of one or more rolled steel sections, which comprises part of the flux path. Because of the high fluxes in a mill-motor it is undesirable to provide unnecessary openings of any kind in this stator-frame so that the frame is substantially continuous along its peripheral area, except for such holes that are filled by bolts for securing to the stator-frame, completely about its circumference, a plurality of oblong main poles l2 and oblong commutating poles or interpoles H in alternating relation, the poles extending for substantially the full length of the armature core subsequently described. The poles are spaced circumferentially and provide relatively longitudinal ventilating stator air-passages l6 between adjacent poles. Such air-passages are circumferentially relatively narrow and extend axially for the full length of the poles, each air-passage having a side in open communication with an air-gap l8 between the rotor and stator. In mill-motors the number of pairs of poles used is generally large, being from three to fourteen pairs of poles and above, twelve pairs of poles being used in the illustrative 7000 H. P. machine. Distributed compensating windings 20 are fastened in the pole faces of the main poles l2, their end turns being formed to minimize resistance to air-flow at the ends of the stator air-passages I6.

The rotor 4 comprises a shaft 22 which has keyed or otherwise secured thereto an open armature spider 24 which comprises a hub 26 from which a plurality of sets of radially directed circumferentially spaced arms 28 extend, the sets being axially spaced. The outer ends of the arms 28 have integral circumferential connecting pieces 30 so that each set is in the nature of a spoked wheel. Adjacent axially aligned arms 28 of the different sets are interconnected by webs 32 provided with large openings through which ventilating air may freely pass. Such armature spiders are common in which the ventilating air can pass freely through and between the rotor air-spaces or air-passages 34, comprising the open spaces between the arms 28 and in the webs 32.

An annular laminated magnetizable armature core 36 fits on and is secured in any suitable manner to the outer portion of the spider 24, the core comprising a plurality of axially relatively short core-sections 38, each section comprising a plurality of punchings or laminations in a bundle or stack, the core-sections or stacks being separated by suitable fingers to provide radial ventilating passages or ducts 40 through which air may pass from the rotor air-spaces 34 into the air-gap l8 and stator air-passages l6. In high-power millmotors such stacks customarily have a length of about 2 /2 to 3 inches and the ducts about 14; inch, axially.

The armature core 36 includes a plurality of outer longitudinal slots 4| for receiving the active coil-sides of an armature winding, indicated in its entirety by the reference numeral 42, the coils having end-turns 44 at each end of the rotor. Mill-motors being slow-speed machines hav ing low frequencies in the armature conductors, deep slots are permissible without undu heating or poor commutation, the deep slots permitting more copper to be disposed therein for the armature conductors, and thus more motor output power can be obtained. In the illustrative 7000 H. P. motor, the armature core laminations had external and internal diameters of 144 inches and 128 inches respectively, there being 12 circumferentially equally spaced slots per pole, each slot having a width of about .5 inch and a depth radially of about 2.5 inches.

The rotor 6 further comprises at its front end, an annular-like commutator or commutator-assembly 4-6 which is suitably fastened to the front end of the spider 24 by any suitable means which may include a perforated flat annular ring 48 secured to the spider 24 and a plurality of spaced gusset-shaped supporting plates 56 fastened edgewise to the inner side of a tubular commutator bush 52. The bush 52 is spaced radially outward from the shaft 22 to provide an inner commutator air-passage between them through which ventilating air may flow. This inner commutator airpassage may be of longitudinal annular form or may consist of a plurality of air-lanes between the commutator and shaft, depending on how it is desired to brace or support the commutator; the commutator air-passage being in opeh communication with the air-spaces in the spider 24, forming a continuous ventilating path with the rotor air-passages 34. In the illustrative 7000 H. P. motor, the shaft diameter at the commutator was 34 inches, and the inside commutator-diameter about 101 inches, thereby providing a relatively large commutator air-passage. Mill-motors as a rule require a large diameter shaft because of the power that must be transmitted at relatively low speeds with correspondingly high torques, so that an adequate inner commutator air-passage is desirable for double-ended ventilation of the type herein described. Since, as will be subsequently described, forced ventilation is utilized and means provided to control the rela tive amounts of incoming air at each end, the radial size of the inner commutator air-passage can be varied, but it should not be so small as to introduce an objectionable resistance to air-flow.

The commutator also comprises a, plurality of individually insulated current-collecting commutator bars 54 and spaced radial air-insulated necks 56 to which the terminus ends of the armature coils are conductively connected in proper relation. A brush rigging 58 for holding the brushes which ride on the commutator bars is suitably supported by the stator 2.

At its rear end the spider 24 suitably carries an armature end ring 60 having spaced lugs 62 around its periphery for supporting end-turn supporting-ring 64. A bafiie means 66, secured to the supporting ring 64 and to a small ring 68 on the spider 24, aids in directing ventilating air, as will be described hereinafter.

The mill-motor 2 further includes end bells or housings, comprising a front end bell or housing [0 and a rear end bell or housing 12 which contribute to the guiding of the air-flow through the motor. In actual practice, the end housings are preferably formed of sections which can be readily disassembled and removed for access to the inside of the mill-motor.

In the described embodiment, the front end housing Hi comprises a substantially flat annular-like end disk 74 having an outer periphery which is circular except for the lack of a lower segment bounded in part by a chord 16. The end disk 14 is provided with 'an opening 18 which is substantially circular except for a small segment 80, which is part of the end disk and is bounded in part by a chord 82 that is substantially parallel to the chord It. A short axially-directed round tubular shroud 84 is attached to the end disk 14, the shroud having substantially the same diameter as the inner central opening 18 and following its curved periphery, having a portion between the chords [6 and 82. The shroud 84 has a diameter which is only slightly less than that of the commutator bush 52. The end housing 10 further comprises a substantially axially directed outer sheet steel tubular member 86 which extends from the outer curved periphery of the .end disk 14 to the periphery of the stator-frame ID. The bottom portion of the end housing in has an outlet or discharge opening 88 which extends axially inward from the outer chord 16, the opening bein bounded by this chord, longitudinally extending sides 90 of the tubular member 86 and a lower are 92 of the tubular member 86, of a size which would be about that subtended by the chord 16 on the circumference of the disk 14, the arc being spaced from the stator-frame side of the end housing 10 and the disk I4.

The rear end housing 12 of the described embodiment is somewhat similar to the front end housing 10, being provided with an end disk 94 and an outer round tubular member 96. The end disk 94 has an inner central opening 98 and two chords, similar to the chords [6 and 82 of the front end housing 10. The end disk 94 also has an outlet or discharge opening I00 bounded by the lower chord, longitudinal sides I02, and a lower arc I64 intermediate the ends of the tubular-member 96. A round tubular shroud extends inwardly from the end disk 94, followgrgg the curved periphery of the central opening The axial length of the outer tubular member 86 of the front-end housing 10 is longer than that of the outer tubular member 96 of the rear end housing 12, while the shroud 84 of the front end housing is considerably less in length than the shroud I06 of the rear end housing. The space between the edges of the front-end shroud 84 and the bush 52 provides a circumferential, radial-flow air-passage I08 having an air-flow capacity determined by the diameter, spacing and size of the shroud 84. The rear-end shroud I06 cooperates with a round tubular member H0 of the bafiie means 66 to provide a guided air inlet to the rotor air-spaces at the rear end of the mill-motor. The baflle means 66 also comprises an annular radial wall II2 which extends outward from the tubular member H8 to the end-turn supporting-ring 64. The facing edges of the shroud I86 and the tubular member I I8 may be spaced slightly to provide a very narrow radial-flow air-passage II4 to the rear portion of the armature coil rear end turns; and the tubular member II8 may be provided with a. plurality of small holes II6 through which a small portion of air may flow outwardly to the rear end turns 44 of the armature coils.

The front end housing I8 cooperates with the outer surface of the commutator-assembly or commutator 48 and the front ends of the statorframe and pole pieces to provide a front header or air-receiving chamber H8 having the outlet 88 for its discharge opening; and the rear end housing I2 in cooperation with the shroud I88, baffle means 88, and the rear of the stator-frame and pole pieces forms a somewhat annular rear header or air-receiving chamber I28 having the outlet I88 as a discharge opening.

In the described embodiment, the mill-motor I is supported on a foundation which includes a pit portion I22 for the lower part of the millmotor. The foundation carries the bearing means 8 above the floor level I24 thereof, each bearing means being supported separated from the stator, longitudinally outward from the associated end housing so as to reduce the restriction interposed to incoming air to the central openings of the end housings. Suitable sealing means are provided to air-seal the pit-portion I22 which is about the portion of the motor therein. This sealing means may comprise a to seal the transverse spaces between the pitportion and the motor. A conduit means I32 connects to the pit-portion I22 and leads to a draft-inducing blower means diagrammatically indicated as a fan-propeller I34 in the conduit means I32.

When the blower means I34 is started, a subatmospheric suction pressure, (about 4-6 inches of water in an actual embodiment such as described) is established in the pit-portion I22 so that air is drawn into the mill-motor 2 at each of its ends. The air-flow through the motor is' approximately shown by the arrows in Fig. 1.

A small portion of the incoming air flowing through the front central opening 18 passes between the shroud 84 and the commutator 46, sweeping over the front portion of the commutator and then mingling with the other air in the front air-receiving chamber II8. Most of the air which enters the motor at the front end passes through the commutator-assembly airpassage into the front portion of the spider 24. A small portion of the air entering this front portion of the spider turns upwardly and flows through the armature coil front end-turns 44 and between the commutator bar necks 56 into the air-receiving chamber H8. The main portion of the air flowing into the front end of the spider 24 turns outwardly and passes out radially through the rotor ducts 48 into and through the front portion of the air-gap I8 and the stator air-passages I8, reversing its original entering flow-direction, and finally flows into the front air-receiving chamber I I8.

At the rear end of the motor, the incoming air flows in a generally similar path. A preponderantly greater portion of this incoming air flows through the shroud 84 and the tubular member II8 into the rear end of the spider 24, then turns and passes radially outward into the air-gap I8 and stator air-passages I8, via the ducts 48 which are at the rear portion of the rotor. The air then flows into the rear header or air-receiving chamber I28. It is apparent that ventilating air flows in opposite directions out of the air-gap and each stator air-passage.

Most of the air discharging to the air-receiving chambers flows through the longitudinal stator air-passages I6, although some of the air flows into the air-receiving chambers from the air-gap For ventilating the armature coil rear endturns 44, a generally circular gap I36 may be provided outwardly of the baflle means 86, through which a portion of the gas flows for cooling such end-turns. Additionally, some air will flow to these end-turns through the plurality of small circumferentially spaced apertures IIB. Some of the incoming air at the rear of the end of the motor also will pass through the space II4 between the facing edges of the shroud I86 and the tubular member III]. However, the air-flow through this space can be controlled by a suitable selection for the diameters of the shroud I88 and member II8.

Air flowing in each of the airreceiving chambers flows downwardly therein to the discharge openings 88 for the front air-receiving chamber and I88 for the rear air-receiving chamber; discharged air rlowing in the pit-portion I22 and being drawn into the conduit means I32 by the draft-inducing blower means I34 from whence it may be vented or used in any manner.

The segment 88 on the front end-housing and the corresponding segment on the rear end-housing prevent incoming air flowing at the lower portions of the central openings I8 and 98, from reversing beyond the sealing means I38 and flowing directly into the pit-portion I22,

The ventilating system of the mill-motor 2 may be said to comprise at least two opposed and distinct flow-paths in the motor, the air-flow in axial sectional plane flowing generally along lines in a series of arcs in the nature of nested fiat-laid Us having their legs axial; but the air in each air-flow path-portion mixes. There may be no single radial plane within the rotor where the central curved portions of the outermost arcs of the nested arcs of the two-fiow-paths meet or separate, but obviously baflle means may be inserted in the rotor, if desired.

The shroud I86 and the baffle means I56, at the rear end of the motor, in a sense impose an air-flow restriction at that end of the motor, in comparison to what would be the natural tendency of air to flow into that end of a direct ourrent motor if it were provided with radially open member longitudinally of the motor, the size of the discharge opening 88 can be varied. ,Obviously a similar flow controlling means may be provided for the rear end housing 12 at it discharge opening I00.

The front end housing is somewhat larger than the-rear end housing 12 to accommodate the commutator-assembly 46 and the brush-rigging 58; and the axial length of the front discharge opening 88 is longer than that of the rear discharge opening I00, but the effective length of these openings can, of course, be relatively changed by any suitable air-controlling means functioning in the manner equivalent to the slidable closure member I40.

As an example of an advantage of my invention, the illustrative 7000 H. P. reversible millmotor, embodying the teachings of my invention, had an inertia of about 3,200,000 pound feet which is less than 500 pound feet per horsepower, and which was about half of that of a representative comparable motor of conventional design, both motors being capable of reversing from base-speed full-field in one direction to basespeed full-field in the reverse direction in about .two seconds. The radius of gyration was about .8 of the outside radius.

This improved 7000 H. P. motor had an armature core diameter of 144 inches and an armature core length, including ducts, of '70 inches, such length and diameter being respectivel greater and less than the length and diameter of the rep resentative comparable machine. The rotor copper weight was decreased by over about primarily bcause the end turns of the armature coils were a smaller fraction of the active sides of the coils. The stator copper Weight was also reduced but not to such a significant extent.

While I have described my invention with special reference to a large-power reversible millmotor as illustrating a preferred embodiment in detail, it is obvious that the invention has broader application to dynamo-electric machines of the commutator type, including machines having higher and considerable less power; and that many modifications may be utilized, and equivalents substituted within the spirit of the invention; and while I have designated air as the ventilating medium, it is obvious that any other gaseous atmospheres may be used, if desired, and which I intend shall be included in the term flair. V

I claim as my invention:

1. A dynamo-electric machine in the size-range from several horsepower per R. P. M. up to a few hundred horsepower per R. P. M., comprising the combination of a stator member having a substantially air-impervious stator-frame, a rotor member separated from said stator frame by an air-gap, a current-collecting device at one end of therotor member, said rotor member having a relatively large shaft and axially extending ventilating passages disposed near the rotor shaft and a plurality of axially spaced radial ventilating ducts extending from said rotor axial ven-' tilating passages to the air-gap of the machine, the stator member being so restricted as to its ventilating capacity as to be a limiting factor in determining the rating of the machine in regard to the possibility of passing the necessary coolinggas axially along the air-gap and the stator memher, the machine being of such slow speed that an external gas-circulating means is necessary in order to force the ventilation, the current-collecting means being of such size and nature as to cause a serious restriction in the gas-flow passages at that end, as distinguished from the other end, of a rotor member, and external gas-circulating means so applied that substantially all of the ventilating gas is drawn in through the central portions around the shaft at both ends of the rotor member, circulated radially through the radial ventilating ducts of the rotor member, and then directed back axially in both directions through the air-gap and the stator member so that each gas-stream returns substantially to the same end of the machine at which it started, whereby the machine may be built with a materially greater length, and hence smaller diameter and rotational inertia, for a given electrical rating, than if single-flow ventilation has been utilized.

2. In a ventilating system for a dynamo-electrio machine of a type described, said dynamoelectric machine comprising a stator member and a rotor member separated by an air-gap, said stator member comprising a single substantially air-impervious stator-frame and field means supported thereby, said field means providing longitudinal ventilating stator air-passages in communication with said air-gap, said rotor member comprising an armature core having armature winding means, and current-collecting means at one end of said rotor member, said rotor member being provided with open air-spaces for centrally receiving incoming ventilating air from both ends thereof, said rotor member having outer ducts which discharge into said air-gap about the outer surface of said armature core, whereby air flows into said stator air-passages; the combination of a first air-directing means comprising a shroud at one end of said stator for causing incoming air at said one end to flow centrally into said rotor air spaces, a second air-directing mean at the other end of said stator cooperating with said current-collecting means for causing incomingair at said other end to flow centrally into said rotor air-spaces, a first air-receiving means outwardly around said first air-directing means, and associated therewith to provide an outer air-collecting chamber at said one end of said dynamoelectric machine, a second air-receiving means outwardly around said second air-directing means and associated with the outside of said currentcollecting means to provide an outer air-collecting chamber at said other end of said dynamoelectric machine, said air-collecting chambers having discharge opening means for discharging air from said air-collecting chambers, and means for forcing air through said dynamo-electric machine to cause substantially all of the motor ventilating air to enter substantially longitudinally at both ends of said rotor member, flow outwardly in said rotor member and then in said stator airpassages in reverse longitudinal directions, whereby spent air flows into said air-collecting chambers.

3. A motor of the type described having, in combination, a stator means and a rotor means separated by an air-gap and cooperating for producing rated power-outputs of several hundred to several thousand horsepower at rotor peripheral speeds of less than 5500 feet per minute, said motor being a direct-current motor, said stator means comprising a substantially continuous stator-frame and a plurality of oblong salient .poles secured thereto which are circumferentially spaced to provide restricted longitudinally extending stator air-passages in communication with said air-gap, said rotor means comprising a shaft, a spider secured to said shaft, core structure having armature winding means secured to said spider, said spider including central airspaces about said shaft inside of said core structure, said core structure including a plurality of separated core-sections providing ducts opening into said central air-spaces and said air-gap, a front and a rear end member for said motor, each of said endmembers comprising air inlet means about said shaft and a substantially air-impervious portion extending from the associated air inlet means to said stator-frame, means including a commutator-assembly at the front end of said core structure, cooperating with said front end member to divide the front end of said motor into a central air-opening and an annular-like outer air-receiving chamber, baflle means cooperating with said rear end member to divide the rear end of said motor into a central air-opening and an annular-like outer air-receiving chamber, each of said stator air-passages opening into said air-receiving chambers, whereby a ventilating system is provided for said motor in which air may flow in opposite directions through said central air-openings, in opposite directions through different portions of said rotor air-spaces, in a generally radial direction in said ducts, and in opposite directions in different portions of said stator air-passages.

4. A salient-pole direct-current dynamo-electric machine of a type described, comprising a stator member and a. rotor member separated by an air-gap, said stator member comprising a stator-frame carrying a plurality of oblong salient poles circumferentially spaced to p o i e o itudinally extending stator air-passages in communication with said air-gap, said rotor member comprising a shaft, armature means comprising an annular core structure carried by said shaft outwardly therefrom, there being open main airspaces between said shaft and said core structure in open communication, and a tubular commutator having central air-spaces in open comniuni-' cation with main air-spaces, said core structure having armature winding means having exposed end-turns, said core structure being provided around its periphery with spaced ducts open-ing into said main air-spaces and said air-gap, said main air-spaces including auxiliary air-passages for discharging air to said end-turns and to the rear of said commutator assembly, a front end housing and a rear end housing, each of said end housings haw'ng central opening-means about said shaft for air flow therethrough into said main air-spaces, said front end housing having a portion spaced from said commutator to provide a relatively small air-passage to the front of said commutator, said rear end housing havin means associated with its central opening-means for, in effect, providing an air-passage from the outside or said end-housing to said main air-spaces; said end housings comprising outer air-receiving chambers in communication with said stator airpassages and said auxiliary air-passages.

5. A mill-motor having, in combination, a Sta-- tor member and a rotor member separated by an. air-gap and cooperating for producing rated power-outputs of several hundred to several thousand horsepower at rotor peripheral speeds of less than 5500 feet per minute, said motor being a direct-current machine, said stator member comprising a substantially air-impervious stator-frame having a plurality of salient poles spaced circumferentially therein to provide con tinuous longitudinal stator air-passages in communication with said air-gap; said rotor member comprising an outer armature structure and a commutator about inner rotor air-spaces, said motor being provided with a ventilating system having air-inlet means for directing air-from both ends of said motor into said inner rotor airspaces, through a plurality of spaced ducts in and about said armature structure, and into said airap and stator air-passages, the air flowing in each of said stator air-passages generally axially outward in opposite directions, said ventilating system comprising air-receiving means associated with each end of said stator for receiving air flowing out of said air-gap and said stator airpassages, said air-receiving means being generally outward and separated from said air-inlet means at the associated end, and having air-outlet means, whereby ventilating air may initially enter and may later leave said mill-motor at each end thereof.

6. The mill-motor of claim 5, in combination with 'an external draft-creating means for forcing ventilating air through said mill-motor and means for controlling the relative air-flow flowing through the respective ends of said mill-motor.

'7. A direct-current reversible mill-motor comprising a stator and a rotor having an air-gap therebetween, said stator comprising a substantially air-impervious stator-frame having a plurality of salient poles spaced circumferentially therein, said rotor comprising a shaft, a relative long outer armature-coil receiving magnetic core outwardly spaced from said shaft, having a relative short outer diameter, whereby to provide a relative low inertia, and a commutator-assemly comprising commutator bars outwardly spaced from said shaft, said mill-motor being of a type normally operable so that the maximum peripheral speed of its rotor is below 5500 feet per minute, said mill-motor being provided with a ventilating system having means which causes ventilating air to flow therethrough in air-flow paths including two general flow-paths, said ventilating system comprising inlet-means solely at each end of said mill-motor in proximity to said shaft, intercommunicating rotor air-passages between said shaft, on the one hand, and said magnetic core and commutator bars, on the other, a plurality of axially spaced ducts in and around said magnetic core, said ducts passing through said magnetic core and opening into said rotor air-passages and said air-gap, longitudinal stator air-passages between said salient poles, said stator air-passages being in communication with said air-gap, stationary means providing air-receivingv chambers at each end of said dyamo-electric machine, outwardly of the associated inlet-means, aid stator air-passages having ends respectively discharging air into the associated one of said air-receiving chambers, whereby air flowing along one of said two paths will include air which flows inwardly from one end of said mill-motor, into "a portion of said rotor air-passages associated with said one end, outwardly through a plurality of said ducts in said rotor portion, through said air-gap and along a portion of the stator air-passages which substantially corresponds to the said portion of said rotor at said one end, to the said air-receiving chamber at said one end, and air flowing along the other of said two paths will include air which flows inwardly from the other end of said mill-motor, into substantially the remaining ortion of said rotor air-passages associated with said other end, outwardly through substantially the remaining of said ducts, through said air-gap and along substantially the remaining portion of the stator air-passages, to the said air-receiving chamber at said other end.

8. The mill-motor of claim 7 characterized by including, in combination, means for controlling the relative distribution of the air-flow in said two air-flow paths.

9. A low-speed direct-current reversible millmotor comprising a stator member and a rotor.

therefor, said rotor having a shaft and rotor central air-passage means between said shaft, on one hand, and said armature core and commutator, on the other hand, said armature core having a plurality of axially spaced radial ducts opening into said air-passage means and airgap, stationary front and rear end housings for said mill-motor, each having wall means providing a central opening for directing incoming air into said rotor air-passage means, said wall means of said front end housing being associated with a front end portion of said stator and the outer portions of said communication to provide a front outer air-receiving chamber, said rotor having wall means, said wall means of said rear end housing being associated with a rear end portion of said stator and with said wall means of said rotor, to provide a rear outer air-receiving chamber, each of said stator air-passages having 20 ends at the respective air-receiving chambers.

CLARENCE LYNN. 

